At present, a contact charging method has been put into practical use as one of methods for charging the surface of an electrophotographic photosensitive member electrostatically.
The contact charging method is a method in which a voltage is applied to a charging member disposed in contact with the electrophotographic photosensitive member, to cause micro-discharge at the contact part between the charging member and the electrophotographic photosensitive member and the vicinity thereof to charge the surface of the electrophotographic photosensitive member electrostatically.
As the charging member for charging the surface of the electrophotographic photosensitive member, from the viewpoint of sufficiently ensuring a contact nip between the electrophotographic photosensitive member and the charging member, one having a support and an elastic layer (conductive elastic layer) provided on the support is commonly used.
The elastic layer (conductive elastic layer) often contains low-molecular weight components in a relatively large quantity, and hence such low-molecular weight components may ooze to contaminate the surface of the electrophotographic photosensitive member. In order to suppress this contamination due to the oozing, it is also prevalent to provide on the conductive elastic layer a surface layer having a lower modulus of elasticity than the conductive elastic layer.
As the shape of the charging member, a roller shape is commonly employed. Hereinafter, the roller-shaped charging member is referred to also a “charging roller”.
The contact charging method in widespread use is a method in which a voltage generated by superimposing an alternating-current voltage on a direct-current voltage is applied to the charging member (hereinafter referred to also as “AC+DC contact charging method”). In the case of the AC+DC contact charging method, a voltage having a peak-to-peak voltage twice or more as high as the voltage at which the charging is started is used as the alternating-current voltage.
The AC+DC contact charging method is a method by which stable charging high in charging uniformity can be performed because of the use of the alternating-current voltage. However, insofar as an alternating-current voltage source is used, this method brings about a charging assembly and an electrophotographic apparatus which are large in size and a rise in cost, as compared with a method in which only a direct-current voltage is applied to the charging member (hereinafter referred to also as “DC contact charging method”).
That is, the DC contact charging method is superior to the AC+DC contact charging method in miniaturizing the charging assembly and electrophotographic apparatus and reducing costs.
As a conductive member used in an electrophotographic apparatus, such as the charging member, a conductive member having an inorganic-organic hybrid film having an organosilicon compound is proposed (see, e.g., Japanese Patent Application Laid-open Nos. 2001-173641 and 2004-210857).